A detonator is a device designed to initiate detonation of an external charge of secondary explosive situated downstream therefrom; in order to do that, every detonator contains a small quantity of secondary explosive (100 milligrams (mg) to 1 gram (g)) which needs to be brought to detonation (at least) in its terminal portion starting with energy supplied to the inlet of the detonator from an external source.
In known manner, an optical detonator is a detonator of the type comprising secondary explosive disposed in a cavity, an optical fiber connected at a first end to a source of laser radiation, and a focusing optical interface situated between the other end of the optical fiber and the secondary explosive, and adapted to transmit the laser radiation to the secondary explosive.
In a manner that is entirely conventional in the field of explosives, the term “secondary” explosive is used to designate an explosive that is relatively insensitive, in contrast with “initiating” or “primary” explosives, e.g. lead azide which are very sensitive and thus dangerous.
In low-energy optical detonators (energy less than 10 millijoules (mJ)) that are also of low power (a few watts), the light energy of the laser radiation from a solid laser source in relaxed mode or from a quasi-continuous laser diode (maximum size 1 cubic centimeter (cm3)) is used via an optical fiber for igniting deflagration of the secondary explosive charged at the optical interface.
This heating by absorbing laser radiation via the optical interface is recognized as presenting optical detonators with greater safety in use compared with electrical detonators in which the explosive substance close to the inlet interface is in intimate and permanent contact with a resistive electrical conductor wire that heats when an electrical current passes therethrough and transmits its heat by thermal conduction to the explosive substance coating it, but which can be activated accidentally by unwanted electrostatic discharges or by induced currents due to interfering electromagnetic radiation.
In spite of this undeniable advantage of optical detonators, use thereof poses various problems due to the fact that the secondary explosives used do not absorb light emitted in the near infrared, whether by solid lasers or by laser diodes.
Thus, in order to mitigate that problem, the state of the art teaches doping the secondary explosive optically, i.e. mixing 1% to 3% by weight of ultrafine carbon black (grain size lying in the range 50 nanometers (nm) to 200 nm) with the secondary explosive (grain size close to 3 micrometers (μm)), so that the laser light is absorbed by the carbon black.
Thus, by means of such optical doping, and by focusing the laser light into a spot of diameter lying in the range 50 μm to 100 μm, the energy threshold of the igniting laser is reduced, thereby making it possible to ensure that the explosive composition is ignited thermally even when using laser diodes that deliver nominal power of 1 watt during a period of 10 milliseconds (ms).
Nevertheless, during operational tests for validating the use of detonators in severe operating environments (use in airplanes, missiles, space vehicles, . . . ) and which are performed either after intense thermal shocks (testing at ambient temperature after being subjected for 5 hours to temperatures above 100° C.), or else after thermal cycling (−160° C. to 100° C.), it has been found that laser ignition of the explosive composition that has been optically doped with carbon black is not sufficiently reliable.
This lack of reliability relates most particularly to nitramines (octogen and hexogen) which are the secondary explosives in most common use for these applications.
Crystals of organic secondary explosive have a coefficient of thermal expansion that is much greater (three times to seven times) than that of the materials used for making a detonator (the silica of the optical interface, stainless steel, or Inconel for the charge-containing body). Thus, when the stresses due to thermal shocks are released, cracks appear in the compressed explosive composition in the vicinity of the optical interface, and as a result the distribution of carbon black in the explosive composition is no longer uniform. Consequently, the secondary explosive is no longer adequately coated in carbon black, thereby sharply increasing the ignition energy threshold and reducing the effectiveness of the optical doping.